You can create a config
file in your home .ssh
directory, which can specify a default user, identity file etc and assign it to an alias which you can use to connect.
For example, you could put this into the ~/.ssh/config
file:
Host example HostName example.com User first_last IdentityFile ~/.ssh/example_rsa Port 22 RSAAuthentication yes PubkeyAuthentication yes Host example2 HostName example2.com User last_first # other parameters as needed
Then you can just type ssh example
or ssh example2
on the command line to connect.
So you can set one up for each machine you want to connect to, with the relevant user. Then just use the aliases to connect without having to specify a user. You can also use a pattern to match multiple hosts.
There's more information, and details of what you can specify in a config file in the ssh_config man page.